His 2010 tax return, which he'll post online along with an estimate of his 2011 tax liability, will offer up some information not previously made public.

But his return will probably leave many unanswered questions about the extent of his holdings and how much he has profited from his retirement package from Bain Capital, the private equity firm he founded.

His income: For the first time, Romney will reveal his 2010 income. And the number is likely to be big, since his net worth is estimated to be as high as $264 million.

Among other things, his return should reveal how much he earned in speaking fees and from his book, No Apology: The Case for American Greatness.

But the majority of his income will come from an extensive portfolio of investments.

Likewise, any taxable income he made from investments in his blind trust -- or his wife's -- would show up as well, although the return would not likely show the actual sources of that income, said Roberton Williams, senior fellow at the Tax Policy Center.

His tax liability: How much Romney paid to the IRS will also be news -- as, of course, will be his effective tax rate.

But it may be even lower than 15% if, for instance, Romney had a lot of deductions, which could offset a sizeable chunk of his tax liability.

In addition, his rate may be lower still because his return may understate his income. Here's why: He may not have to report some forms of tax-free income -- such as gains from the sale of a home up to a cap, said Mark Luscombe, principal federal tax analyst at tax information publisher CCH.

On the other hand, his effective rate might be somewhat higher than 15% if a lot of his taxable investment income was generated by holdings that were bought and sold within a year. In that case, his investment income would be taxed at ordinary income tax rates, and probably all of it at his top rate of 35%.

His deductions: Romney almost certainly itemizes his deductions -- meaning he has enough eligible items to deduct that, when combined, exceed the value of the standard deduction.

Among the itemized deductions he's likely to claim are charitable contributions. Romney, a Mormon, told Fox News on Sunday that he regularly tithes 10% of his income to the Mormon church.

And here's another prediction: He is likely to deduct high fees for help preparing his tax returns, since Romney's 1040 almost certainly will not be a TurboTax affair.